Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as a power source for electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (Plug-in HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuels.
Small-sized mobile devices use one or several battery cells for each device. On the other hand, middle or large-sized devices, such as vehicles, use a battery pack having a plurality of battery cells electrically connected to each other because high power and large capacity are necessary for the middle or large-sized devices.
Preferably, a battery pack is manufactured so as to have as small a size and weight as possible. For this reason, a prismatic battery or a pouch-shaped battery, which can be stacked with high integration and has a small weight to capacity ratio, is usually used as a battery cell of the battery pack. In particular, much interest is currently focused on such a pouch-shaped battery, which uses an aluminum laminate sheet as a sheathing member, because the pouch-shaped battery is lightweight, the manufacturing cost of the pouch-shaped battery is low, and it is easy to modify the shape of the pouch-shaped battery.
In order for a middle or large-sized battery module to provide power and capacity required by a specific apparatus or device, it is necessary for the middle or large-sized battery module to be configured to have a structure in which a plurality of battery cells is electrically connected in series to each other, and the battery cells are stable against external force.
Also, the battery cells constituting the middle or large-sized battery module are secondary batteries which can be charged and discharged. Consequently, a large amount of heat is generated from the high power, large capacity secondary batteries during the charge and discharge of the batteries. If the heat, generated from the unit cells during the charge and discharge of the unit cells, is not effectively removed, the heat accumulates in the respective unit cells with the result that the deterioration of the unit cells is accelerated. According to circumstances, the unit cells may catch fire or explode. For this reason, a cooling system is needed in a battery pack for vehicles, which is a high power, large capacity battery, to cool battery cells mounted in the battery pack.
In a middle or large-sized battery pack including a plurality of battery cells, on the other hand, the deterioration in performance of some battery cells leads to the deterioration in performance of the entire battery pack. One of the main factors causing the non-uniformity in performance is the non-uniformity of cooling between the battery cells. For this reason, it is necessary to provide a structure to optimize the shape of a flow channel, thereby minimizing temperature deviation during the flow of a coolant.
Some conventional middle or large-sized battery packs use a battery pack case configured to have a structure in which a coolant inlet port and a coolant outlet port are located at the upper part and the lower part of the battery pack case, respectively, such that the coolant inlet port and the coolant outlet port are directed in opposite directions, and the top and bottom of a flow space extending from the coolant inlet port to a battery module are parallel to each other. In this structure, however, coolant flux in a flow channel adjacent to the coolant outlet port is greatly reduced with the result that temperature deviation between battery cells is high.
In connection with this case, there is disclosed a middle or large-sized battery pack configured to have a structure in which an air guide plane is inclined downward to the side of a battery pack case opposite to battery cells such that the air guide plane becomes closer to the battery cells with the increase in distance between the air guide plane and a coolant inlet port. Specifically, the air guide plane is inclined at a predetermined angle, for example an angle of 15 to 45 degrees, to the side of the battery pack case opposite to the battery cells, and the coolant inlet port is formed in parallel, thereby restraining the occurrence of a phenomenon in which a coolant is excessively introduced into a flow channel adjacent to a coolant outlet port.
However, the inventors of the present application have found that it is not possible to achieve desired cooling uniformity through the above structure, and, in particular, temperature deviation between the battery cells is changed depending upon the position of the coolant outlet port.